Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates

Zr-doped rare-earth molybdates (Nd5.4Zr0.6MoO12.3, Sm5.4Zr0.6MoO12.3, Dy5.4Zr0.6MoO12.3 and La5.8Zr0.2MoO12.1) demonstrate appreciable mixed electron-proton conductivity in the low and intermediate temperature range under wet oxidizing and mild reducing conditions. Proton contribution to their total conductivity decreases as the lanthanide cation radius decreases. Among the samples studied, La5.8Zr0.2MoO12.1 showed the highest total conductivity of about 2.5 × 10− 5 S/cm at 500 °C in wet air. Its impedance spectra did not provide any evidence of the grain boundary contribution, which seems to be an inherent feature of lanthanum molybdates. Exposure of Nd5.4Zr0.6MoO12.3 to wet argon was found to increase its total conductivity by almost one order of magnitude relative to the values obtained in dry argon. Although all the rare-earth molybdates studied in this work became essentially electronic conductors in Ar-5% H2 atmosphere, La5.8Zr0.2MoO12.1 demonstrated much lower propensity to reduction than the rest of the samples.